Date: Wed, 11 Oct 2006 13:57:20 -0700
From: Stephen Hemminger <shemminger@osdl.org>
To: Steven Whitehouse <steve@chygwyn.com>
Cc: "Michael S. Tsirkin" <mst@mellanox.co.il>,
       David Miller <davem@davemloft.net>, linux-kernel@vger.kernel.org,
       netdev@vger.kernel.org, openib-general@openib.org, rolandd@cisco.com
Subject: Re: Dropping NETIF_F_SG since no checksum feature.
Message-ID: <20061011135720.303f166b@freekitty>
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	<20061011150103.GF4888@mellanox.co.il>
	<20061011201138.GA21657@fogou.chygwyn.com>
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On Wed, 11 Oct 2006 21:11:38 +0100
Steven Whitehouse <steve@chygwyn.com> wrote:

> Hi,
> 
> On Wed, Oct 11, 2006 at 05:01:03PM +0200, Michael S. Tsirkin wrote:
> > Quoting Steven Whitehouse <steve@chygwyn.com>:
> > > > ssize_t tcp_sendpage(struct socket *sock, struct page *page, int offset,
> > > >                      size_t size, int flags)
> > > > {
> > > >         ssize_t res;
> > > >         struct sock *sk = sock->sk;
> > > > 
> > > >         if (!(sk->sk_route_caps & NETIF_F_SG) ||
> > > >             !(sk->sk_route_caps & NETIF_F_ALL_CSUM))
> > > >                 return sock_no_sendpage(sock, page, offset, size, flags);
> > > > 
> > > > 
> > > > So, it seems that if I set NETIF_F_SG but clear NETIF_F_ALL_CSUM,
> > > > data will be copied over rather than sent directly.
> > > > So why does dev.c have to force set NETIF_F_SG to off then?
> > > >
> > > I agree with that analysis,
> > 
> > So, would you Ack something like the following then?
> >
> 
> In so far as I'm able to ack it, then yes, but with the following
> caveats: that you also need to look at the tcp code's checks for
> NETIF_F_SG (aside from the interface to tcp_sendpage which I think
> we've agreed is ok) and ensure that this patch will not change their
> behaviour, and here I'm thinking of the test in net/ipv4/tcp.c:select_size()
> in particular - there may be others but thats the only one I can think
> of off the top of my head. I think this is what davem was getting at
> with his comment about copy & sum for smaller packets.
> 
> Also all subject to approval by davem and shemminger of course :-)
> 
> My general feeling is that devices should advertise the features that
> they actually have and that the protocols should make the decision
> as to which ones to use or not depending on the combinations available
> (which I think is pretty much your argument).
> 
> Steve.
> 

You might want to try ignoring the check in dev.c and testing
to see if there is a performance gain.  It wouldn't be hard to test
a modified version and validate the performance change.

You could even do what I suggested and use skb_checksum_help()
to do inplace checksumming, as a performance test.


-- 
Stephen Hemminger <shemminger@osdl.org>
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